Lock-in imaging systems have been utilized to obtain synchronous components from signals having steady-state and time-varying components. This technology has recently been expanded to use in video signal images.
U.S. Pat. No. 4,878,116, issued Oct. 31, 1989 and assigned to the assignee of the subject invention, discloses a vector lock-in imaging system wherein a video camera detects emitted and reflected radiation from the object field and produces the video signal comprising the series of pixels representing the frame of the image. The video signal is digitized and received by a processor which in turn synchronously averages the successive frames as in-phase and quadrature images based on the periodicity of the object field to eliminate unsynchronous noise from the image and to display the image synchronous with the periodicity of the object field.
The vector lock-in imaging system uses an infrared or visible video camera coupled to a real-time image processor and a computer workstation to perform phase-sensitive lock-in detection on all of the pixels of an image in parallel, thus achieving the effect of having 512.times.512 lock-in amplifiers. In that invention the processor multiplies the incoming video signal by the since and the cosine of the reference signal in real time, and averages the two in separate frame buffers to produce in-phase and quadrature images.
The basic limitation to that invention and to a wide variety of high speed video imaging systems is the accuracy to which each pixel of the image is digitized. Typically, this digitization is carried out to 8-bit accuracy, i.e., to one part in 256. The reason for this limitation is the difficulty and expense of maintaining both high speed and high accuracy in the single digitization process.